Circuit board having shielding planes with varied void opening patterns for controlling the impedance and the transmission time

ABSTRACT

A flexible circuit board with specific shielding planes is used for low voltage differential transmission mode circuits. Both the impedance and the transmission time for the transmission line in the circuit board are controlled by shielding planes with varied void opening patterns. Capacitance and slow wave effects related to the combination of void opening patterns and the location configuration related to locations of void opening patterns are used to improve the impedance and transmission timing for the transmission line in the circuit board.

CROSS-REFERENCES TO RELATED APPLICATIONS

This is a division of U.S. application Ser. No. 09/144,005, filed Aug.31, 1998 now U.S. Pat. No. 6,225,568.

FIELD OF THE INVENTION

The present invention relates to a circuit board or a flexible cable(Flexible Printed Circuit Board (FPC), Flexible Flat Cable (FFC), etc.),and more specifically, to a circuit board or a flexible cable withshielding planes, which have varied void opening patterns used tocontrol both the impedance and the transmission time.

BACKGROUND OF THE INVENTION

For high-speed logic circuits in digital computers, applications ofmicrostrip and stripline structures are very often and extensively usedto the interconnection between these circuits. Moreover, for thehigh-speed data transmission rate, the differential mode of transmissionlines with shielding planes is a necessary and important method for lowvoltage (3.3 Voltage) to reduce transmission errors induced by the noiseduring the data transmission. There is a requirement of one hundred ohmimpedance for differential mode of transmission lines. Recently,automatic techniques and equipment are capable of fabricating thesestructures with the controllable impedance and transmitted timing ofsignal paths. However, the asymmetric structure of microstrips has aserious disadvantage because of the weakness of the mechanicalproperties for flexible cables, and it also permits significant levelsof extraneous electromagnetic radiation.

For the symmetric structure of striplines, not only can it enhance themechanical features of flexibility and anti-fatigue for flat flexiblecables, but also greatly reduce effects of electromagnetic radiation.However, due to the symmetric structure of striplines, the additionalground reference plane greatly reduces the impedance of signal pathbecause of the increasing capacitance between the signal path and theadditional ground reference plane. To consider the high flexibility andanti-fatigue with repeated motion for the stripline type of flatflexible cables, the distance between the differential mode of signalpaths and the ground reference plane should be designed to be shorterthan that between the pair of the differential mode of signal paths. Italso implies that the same design method is needed for avoidingincreasing the thickness of the printed circuit board to maintain thedesired impedance at the same time.

Ground planes or other voltage reference planes are positioned in planesparallel to the conductor planes in which the conductors are designed inplane in a flexible cable or a printed circuit board. It aims to controlthe impedance of the conductors and to block the transmission ofelectromagnetic radiation from conductors carrying high frequencysignals. A solid ground plane is generally used in printed circuitboards or flexible cables, but it is inflexible except for the thin filmtype.

Another disadvantage for solid ground plane is that the impedance ofsignal lines may be lower than desired because the large capacitance isbuilt up in the small spacing between the signal lines and a solidground plane or the reference plane. The more we increase the spacingbetween the signal lines and a solid ground plane or the reference planeto reduce the capacitance and thereby increase the impedance of thesignal lines, the thicker and thus less flexible and more likely tobreak with repeated motion. Similarly, a printed circuit board becomesthicker and thus more heavy and more costly to build.

Reference planes having conductive elements formed in a grid have beenutilized in microstrip designs to increase the impedance and to provideflexibility. However, because the grid is not continuous like a solidreference plane, it has been found to be quite difficult to control bothimpedance and transmitted timing of signal lines protected by a gridreference plane with only one pattern.

One of the particular difficulties is to control the impedance offlexible cables and printed circuit boards utilizing grid referenceplanes, especially for stripline type cables with the structure ofturns. Generally, when the orientation of a signal line needs to bechanged by, for example, 90 degrees or the like, the turn is notincorporated into the signal line with a single 90-degree turn. Rather,the change in orientation is generally implemented with a curve suchthat the orientation of the signal line varies continuously from itsoriginal orientation to the new orientation. It is likely that thesignal line will have different alignment with the conduction of theupper or the lower grid with both grids at various points in the turn.Such alignment causes a significant variation in impedance at suchpoints and causes a substantial impedance discontinuity.

The impedance of microstrip and stripline construction is determined bythe signal conductor width, the separation of the conductor from thereference planes, the dielectrics that surround the conductor and, to alesser degree, the thickness of the conductor. However, such traditionalmethods of determining impedance in striplines and microstrips imposetoo many constraints on the designer. For example, the impedance needsto be twice of 100 ohms for the differential mode transmission lines.One way of obtaining such high impedance using existing technology is toincrease the separation between the signal conductor and the referenceplane. However, this would require the use of a thicker flexible cablewith the less flexibility and anti-fatigue, different dielectricconstant materials surrounding the conductor, or expensive printedcircuit boards of nonstandard thickness. Such nonstandard printedcircuit boards are not only expensive to implement, but also undesirablein many applications due to their thickness.

There is a disadvantage associated with traditional microstripconstruction with high-speed transmission rate in that it generates bothforward and reverse crosstalk, which can seriously deteriorate signalquality. Crosstalk is the effect of coupling the signal of one channelinto another. Crosstalk may arise from a number of sources, one of whichis the unbalance of cable parameters, in particular, the capacitance andinductance between conductors. Therefore, this unbalance may result in aserious coupling from the signal of one conductor into another, and suchunbalance is generally aggravated when a conductor is exposed tononhomogeneous media, as is the case with traditional microstripconstruction.

Solid surface conductors in traditional microstrip construction are mostlikely to radiate high levels of electromagnetic radiation to interferewith the functioning of surrounding electronics. Conversely, extraneousradiation may also affect the operation of surface conductors. Intraditional microstrip construction with high-speed transmission rate,the surface of the conductor is free to radiate into the cavity of thesystem enclosing the circuit board, so it is difficult to provideadequate shielding. It implies that the structure of stripline is neededto reduce the containment of such radiation. However, desired highimpedance conductors of stripline construction is very difficult toimplement without drastically increasing the separation betweenreference planes and conductors. Such an undesired increase in thicknesswould cause problems for the case of thin circuit boards needed innotebook computers or other standard circuit board needed to reduce thecost.

Flexible reference planes are needed for a stripline type flexible cableto have the capability of thousands of flexures, to achieve desiredimpedance and transmitted timing that permit transfer of the signalswithout degrading the signal quality, and to provide an acceptableshielding capability.

Due to the slow wave effect, the transmitted time on the solid plane isfaster than that of the shielding with void opening patterns. When thedifference among the length of the transmission line is large enough tohave timing effect for high-speed transmission rate, the traditionalmethod is to add extra equivalent length for the short transmissionlines at some place to compensate for this effect. But this traditionalextra length compensation may induce an undesired electromagneticradiation due to the discontinuity of the impedance.

Even though the transmission lines have the same length, sometimes, itis necessary to control the transmitted timing to avoid the significantharmonic mode of the high-speed transmission rate. The transmitted timeis related to the dielectric constant of the surrounding material, theslow wave effect due to the shielding pattern, and the compatible longlength of transmission line. If it equals the significant harmonic modeof the high speed of transmission rate, the harmonic mode of this signalwill bounce back and forth between two ends of the transmission line todeteriorate the signal transmission.

SUMMARY OF THE INVENTION

The present invention is related to a flexible cable (Flexible PrintedCircuit Board (FPC), Flexible Flat Cable (FFC), etc.) and a printedcircuit board with shielding planes for transmission lines (preferably,low voltage differential transmission mode circuits). According to thepresent invention, both the impedance and transmission time for thetransmission line in the cable can be controlled by shielding planeswith varied void opening patterns. Capacitance and slow wave effectsrelated to the combination of void opening patterns and the locationconfiguration related to locations of void opening patterns are themajor factors to consider.

Two major categories of varied void opening patterns are used. The firstone is to design the shielding planes with a set of varied void openingpatterns, and the second one is a combination of a set of varied voidopening patterns (preferably, a fixed void opening pattern) and a set ofsmall predetermined configuration of solid patterns.

It is also intended to design the transmission time and impedance withthe consideration of the mechanical flexibility of the flexible cable orthe thickness of the printed circuit board.

Varied void patterns are applied on the shielding plane to generate slowwave effect and to reduce the capacitance of the transmission lineswhich can control the transmission time and increase the impedance atthe same time for matching the requirements.

Thin film type conductive elements such as silver paste on shieldingplanes are connected and designated as the only ground paths for thetransmission lines. Suitable resistance type impedance is provided toreduce the undesired harmonic mode effect which bounces back and forthbetween two ends of the transmission line. At least one ground path(preferably one) for the DC power is added to reduce the powerconsumption of silver paste. During the turn of the cable, the crosstalkbetween the transmission lines is blocked by use of via located atsuitable position with suitable spacing.

Therefore, the object of the present invention is to provide a printedcircuit board or a flexible cable that comprises a first shielding planewith a predetermined shielding configuration, which consists ofconductive elements with a first set of predetermined varied voidopening patterns and a first predetermined location configuration. Thepredetermined location configuration is related to locations of variedvoid opening patterns. There are two predetermined locationconfigurations: the first one is not related to curves of the signallines, and the second one is related to curves of the signal lines. Theprinted circuit board or the flexible cable further comprises a secondshielding plane with a second predetermined shielding configuration,which consists of conductive elements with a second set of predeterminedvaried void opening patterns and a second predetermined locationconfiguration.

Another object of this invention is to provide a method for simplifyingthe design of the impedance and transmission time with the same firstand second location configurations. The method may have the same set ofvaried void opening patterns and the same location configuration, butthe first shielding plane is the mirror image of the second shieldingplane. The identical shielding plane is used for further simplifying thedesign of the impedance and transmission time. A conducting element ispositioned between the first and the second shielding planes to form thesignal conductor.

Another aspect of the present invention is to provide a printed circuitboard or a flexible cable that comprises a first shielding plane with apredetermined shielding configuration, which comprises conductiveelements with the combination of a fixed void opening pattern (or a setof varied void opening patterns) and a small predetermined portion ofsolid pattern. The printed circuit board or the flexible cable furthercomprises a second shielding plane having a second predeterminedconfiguration. A conducting element is positioned between the first andthe second shielding to form the signal conductor.

Another aspect of the present invention is a method to select variedvoid opening patterns, which induce slow wave effects to compensate forthis timing effect with the less undesired electromagnetic radiation.

Another aspect of the present invention is a method to select thepreferable locations and preferable orientation for the small portion ofsolid shielding.

Another object of this invention is to provide a printed circuit boardor a flexible cable with the microstrip structure that comprises ashielding plane with a predetermined shielding configuration, whichconsists of conductive elements with a set of predetermined varied voidopening patterns and a predetermined location configuration.

Other object of this invention is to provide a printed circuit board ora flexible cable with the microstrip structure with a shielding plane.The shielding plane has a predetermined shielding configurationcomprising conductive elements with the combination of a fixed voidopening pattern (or a set of varied void opening patterns) and a smallpredetermined portion of solid pattern.

Other features and advantages of the present invention will becomeapparent from the following description which refers to the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a plan view of a shielded circuit board as the firstembodiment in accordance with the present invention.

FIG. 2 depicts an enlarged view of a section denoted by 2 in FIG. 1.

FIG. 3 depicts a cross-sectional elevational view of the presentinvention taken along the line 3—3 in FIG. 2.

FIG. 4 depicts an enlarged plan view of a section denoted by 4 in FIG. 1showing a method of making a 90-degree turn in a signal conductor.

FIG. 5 depicts a cross-sectional elevational view of the presentinvention taken along the line 5—5 in FIG. 4.

FIG. 6 depicts a plan view of a shielded circuit board as the secondembodiment in accordance with the present invention.

FIG. 7 depicts an enlarged view of a section denoted by 7 in FIG. 6.

FIG. 8 depicts a cross-sectional elevational view of the presentinvention taken along the line 8—8 in FIG. 7.

FIG. 9 depicts a plan view of a shielded circuit board as the thirdembodiment in accordance with the present invention.

FIG. 10 depicts an enlarged view of a section denoted by 10 in FIG. 9.

FIG. 11 depicts a cross-sectional elevational view of the presentinvention taken along the line 11—11 in FIG. 10

FIG. 12 depicts an enlarged view of a section denoted by 12 in FIG. 9.

FIG. 13 depicts a cross-sectional elevational view of the presentinvention taken along the line 13—13 in FIG. 12.

FIG. 14 depicts a plan view of a shielded circuit board as the fourthembodiment in accordance with the present invention.

FIG. 15 depicts an enlarged view of a section denoted by 15 in FIG. 14.

FIG. 16 depicts a cross-sectional elevational view of the presentinvention taken along the line 16—16 in FIG. 15.

FIG. 17 depicts a plan view of a shielded circuit board as the fifthembodiment in accordance with the present invention.

FIG. 18 depicts an enlarged view of a section denoted by 18 in FIG. 17.

FIG. 19 depicts a cross-sectional elevational view of the presentinvention taken along the line 19—19 in FIG. 18.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to FIG. 1, a plane view of a shielded circuit board 30 asthe first embodiment in accordance with the present invention isillustrated.

The circuit board 30 may electrically connect the base and the movabledisplay screen of a notebook computer for low voltage (3.3 Voltage)differential mode with high-speed transmission rate (455 MHz) and highimpedance (100 ohms). The circuit board 30 with a turn has effect ofvaried transmission timing, and the circuit board with compatible length(around 32 cm) has effect with the second mode of harmonics (910 MHz) ofthe transmission frequency 455 MHz. The impedance is designed around 100ohms. Time Domain Reflector (TDR) is used to measure the transmittedtiming with different varied opening patterns, and the measuredtransmission time is in the range between 1.8 nsec (for solid referencepattern) and 2.5 nsec (for grid pattern), that is, the transmission timeis between 0.9 nsec and 1.25 nsec.

Therefore, the goal of the present invention is to control both theimpedance and the transmitted timing at the same time by using shieldingplane with varied void opening patterns. And varied void openingpatterns can be categorized as the following two major categories: thefirst one is to design the shielding planes with a first set ofpredetermined varied void opening patterns, and the second one is acombination of a set of predetermined fixed void opening patterns(preferably one) and predetermined small portion of solid patterns.These configurations of void opening patterns have not only controllableimpedance but also varied slow wave effects to compensate for variedtiming effect with less undesired electromagnetic radiation.

The circuit board 30 is classified as the second category, so thepredetermined small portion of solid shielding, as shown in a sectiondenoted by 4 of FIG. 1, is the area of the conductive element withoutany void opening pattern. It comprises only solid pattern. Thepreferable locations of the solid patterns are at the turn of thecircuit board 30, both ends of the cable, and the place of the cableneeded to be folded because discontinuity of the impedance at theselocations normally can not be avoided. The preferable void openingpattern is symmetric such as circle, square, ellipse, and rhombus. Themost preferable one is circle because it is symmetric in any direction.

Another aspect of the present invention uses the shielding planes 28, 40as the only ground for the differential mode of transmission lines 60,62, 64, 66, 68, 70 as shown in FIGS. 1 and 2. That is, in the middlelayer of the cable there is no ground conductor all the way between bothends of the cable. This can greatly reduce the extra need of via for theground conductors positioned all the way in the middle layer and betweentwo ends of the cable. Therefore, further aspect of the presentinvention is a method to design better mechanical features of repeatingflexing and to prevent electromagnetic radiation as we compared with thetraditional method. A further aspect of the present invention by usingthe shielding planes 28, 40 as the only ground for the differential modeof transmission lines 60, 62, 64, 66, 68, 70 is to have better slow waveeffects to control the transmitted timing. The usage of thin film typeconductive elements can also offer suitable resistance type impedance toreduce the undesired harmonic mode effect which bounces back and forthbetween two ends of the transmitted line 60, 62, 64, 66, 68, 70.

Most of the time, the DC power 88, as shown in FIG. 1, is from one endof the circuit board 30 to the other, and the position of it is mostlikely in the middle layer of the flexible circuit board 30. The thinfilm type conductive element (i.e. silver paste) is designated as theonly ground for the high-speed of differential mode of transmissionlines 60, 62, 64, 66, 68, 70. Therefore, there is one path all the wayin the middle of the flexible circuit board 30 as the ground 54 for theDC power 88 parallel to the differential mode of transmissions lines 60,62, 64, 66, 68, 70, as shown in FIGS. 1 and 2. This ground 54 isdesigned only for the DC power 88 and it is not for the high-speedtransmission lines 60, 62, 64, 66, 68, 70. So the preferable location isas far as possible away from the transmission lines 60, 62, 64, 66, 68,70 as shown in FIGS. 1 and 2.

The circuit board 30 of the present invention comprises as illustratedin FIG. 2: an upper shielding plane 28 comprising a conductive elementwith predetermined configuration of a circular void opening pattern 82;a lower shielding plane 40 comprising a similar conductive element withpredetermined configuration of the same void opening; and differentialmode of signal conductors 60, 62, 64, 66, 68, 70, wherein all thevirtual ground 80 of these pairs are aligned in line with respect to thecenter of the void opening patterns 82. The preferred conductive elementin both of the shielding planes 28, 40 is thin film type conductiveelements, such as sliver past, with mechanical features of repeatedflexing and anti-fatigue, and it is easy to implement a complicatedshielding pattern required by the present invention if printed silverpaste is used.

The upper shielding plane 28 is in alignment with the lower shield planewith respect to the center of the void opening patterns 82 withpreferable no offset. D2 is the spacing along the virtual ground 80 ofdifferential mode of signal conductors 68, 70 between two void openingson the upper plane 28. Conversely, D3 is the length of a void opening onthe upper plane 28 along the virtual ground 80. After we test manylocations of the virtual ground 80, the applicable locations are wherethe pair of differential mode signal conductors 68, 70 can be locatedboth inside the circular void opening pattern to have slow wave effects.It depends on the curve of the signal conductor, and the length of D1and D2 as shown in FIG. 2. The preferred configuration of circle typevoid opening patterns is to have D2=D3 for the cable without changingthe orientation. For the circuit board 30, the virtual ground 80 locatedat the left of the center of circle is best to have equal timing forsignal conductors 68, 70 because of an increasing solid spacing D2 forthe signal conductor 68 and a decreasing spacing D2 for the signalconductor 70. Capacitance effect related to the void opening pattern inthe shield planes 28, 40 and the slow wave effect can be taken intoaccount to design the impedance and the transmitted time from one end ofthe flexible cable to the other end of the cable, respectively, at thesame time.

If the distance D1 is designated the largest dimension of the openingsin both of the shielding planes 20, 40, then in order to have effectivebarrier to the emission of electromagnetic radiation, the distance D1 ispreferably designed to be less than {fraction (1/20)} of the size of thesmallest expected wavelength of the signal traveling through the signalconductors 60, 62, 64, 66, 68, 70. Therefore, both of the shieldingplanes 28, 40 are effective shielding planes.

In order to more fully understand the following description, it ishelpful to establish an X, Y, and Z coordinate system for the drawingFIGS. 2-5. The X and Y axes lie in a horizontal plane as shown in FIG.2. Both of the upper and lower shielding planes are parallel to thehorizontal plane. Similarly, the differential mode of transmissionconductors 60, 62, 64, 66, 68, 70 are in a plane which is between andparallel to the upper and lower planes 20, 40. The principalorientations of the differential mode of transmission conductors 60, 62,64, 66, 68, 70 are parallel to the Y axis, as illustrated in FIG. 2, orparallel to the X axis when a 90 degree turn is needed, as shown in FIG.1. The Z axis is perpendicular to both of the X and Y axes, as shown inFIG. 3.

The location configurations related to void opening patterns are thefollowing: the first one is related to curves of the signal lines, andthe second one is not related to curves of the signal lines. Cables 30,130, 372 as shown in FIGS. 1, 6, 17, respectively, are belonging to thefirst one, and cables 210, 310 as shown in FIGS. 9 and 14, respectively,are belonging to the second one.

FIG. 3 depicts a cross-sectional elevational view of the presentinvention taken along the lines 3—3 in FIG. 2. There are solder maskerlayers on both sides to protect the conductive element, and thestructure is summarized as the following description. The first layer isa very thin solder masker. The second layer is a shielding plane 28 witha conductive element, which is defined as the first plane with the firstpredetermined shielding configuration. The third layer is PI(Polyamide), PET or equivalent flexible material.

The fourth layer is an adhesive layer. The fifth layer is a signalconductor (copper) layer. The sixth, the seventh, the eighth and theninth layers are as the same as the fourth, the third, the second, andthe first layers, respectively. The eighth layer is a shielding plane 40with a conductive element, which is defined as the second plane with thesecond predetermined shielding configuration. The third and fourthlayers can be combined by using a special adhesive type flexiblematerial to reduce the thickness, and so do the sixth and seventhlayers.

Flexible cable with 6 mil has been produced in accordance with thepresent invention.

FIG. 4 depicts an enlarged plan view of a section denoted by 4 in FIG. 1showing a method of making a 90-degree turn in a signal conductor. Solidpatterns 72, 74 are located at the turn of the cable 30, and solidpattern 72 is located at the upper plane and solid pattern 74 is locatedat the bottom plane, as shown in FIGS. 4 and 5. There are vias 50located at the turn of the cable 30 between two pairs of signalconductors 64, 66 and 68, 70 to block the crosstalk due to discontinuityat the turn. These vias are not necessary for large spacing between pairof signal conductors 64, 66 and pair of signal conductors 68, 70 becauselarge spacing between these two pairs 64, 66 and 68, 70 may not havesignificant crosstalk. If the accuracy of impedance is required in theabove solid pattern area, then different and compatible widths of thesignal lines for varied length of the transmission lines are needed tohave the same impedance. Therefore, the longer the length of thetransmission lines 64, 66, the less the width of the signal lines 64, 66is needed to have the same impedance. It implies that, during thesections 72, 74, the width of signal conductor pair 64, 66 is less thanthat of signal conductor pair 68, 70 as shown in FIG. 5.

With reference to FIG. 6, a plane view of a shielded flexible circuitboard 130 as the second embodiment in accordance with the presentinvention is illustrated. The first and the second varied void openingpatterns are circles 160, 162, 164 with varied diameters as shown inFIGS. 6 and 7. The location configurations of the varied opening circles160, 162, 164 are related to the orientation curves of the signalconductors 90, 92, 94, 96, 98, 100 as shown in FIGS. 6 and 7.

Varied void opening patterns 160, 162, 164 induce slow wave effects tocompensate for timing effect with less undesired electromagneticradiation as compared with the addition of the extra equivalent lengths.The easiest preferable way to implement the varied void opening patterns160, 162, 164 is to design them with compatible percentage of void openareas for varied length of transmitted lines 90, 92, 94, 96, 98, 100. Asshown in FIGS. 6 and 7, the longer the length of the transmission lines90, 92 is, the less the percentage of void area of pattern 160 is. Thatis, the less the percentage of void area of pattern 160, the less thetransmitted time of the signal conductors 90, 92 is. If the accuracy ofimpedance is also required in the above varied void opening area, thendifferent and compatible widths of the signal lines for varied length ofthe transmission lines are needed to have the same impedance. Therefore,the longer the length of the transmission lines 90, 92 with smallerpercentage of void area pattern 160 is, the less the width of the signallines 90, 92 is needed to have the same impedance. It implies the widthof signal conductor 90 is less than that of signal conductor 94 as shownin FIG. 7.

FIG. 8 depicts a cross-sectional elevational view of the presentinvention taken along the line 8—8 in FIG. 7.

With reference to FIG. 9, a plan view of a shielded flexible circuitboard as the third embodiment 210 in accordance with the presentinvention is illustrated. The circuit board 210 with a turn has effectof varied transmission timing, and the circuit board 30 with compatiblelength (around 32 cm) has effect with the second mode of harmonics (910MHz) of the transmission frequency 455 MHz. The circuit board board 210has a combination of a square void opening square pattern 250 and apredetermined small portion of solid patterns 274, 272 located at theturn of the circuit board 210, as shown in a section denoted by 12 ofFIG. 9. The location configurations of the void opening square pattern250 are the same and not related to the orientation curves of the signalconductors 260, 262, 264, 266, 268, 270, as shown in FIGS. 9 and 10.FIG. 10 depicts an enlarged view of a section denoted by 10 in FIG. 9,and FIG. 11 depicts a cross-sectional elevational view of the presentinvention taken along the line 11—11 in FIG. 10.

FIG. 12 depicts an enlarged view of a section denoted by 12 in FIG. 9with the solid pattern 272 absent. FIG. 13 depicts a cross-sectionalelevational view of the present invention taken along the line 13—13 inFIG. 12. If the accuracy of impedance is needed, then the width ofsignal conductors 264, 266 gradually becomes small for matching therequirements of impedance because of increasing of capacitance by solidpatterns 272, 274. That is, the width of signal conductors 264, 266within the area of solid patterns 272, 274 is less than that of signalconductors 268, 270, as shown in FIGS. 12 and 13.

With reference to FIG. 14, a plan view of a shielded flexible circuitboard as the fourth embodiment in accordance with the present inventionis illustrated. The circuit board 310 has varied square void openingswith location configuration which is unrelated to the curve of thecircuit board 310. FIG. 15 depicts an enlarged view of a section denotedby 15 in FIG. 14. FIG. 16 depicts a cross-sectional elevational view ofthe present invention taken along the line 16—16 in FIG. 15. It shouldbe noted that void opening square patterns 354, 356, 358 have variedopening squares. Varied void opening square patterns 354, 356, 358induce slow wave effects to compensate for timing effect with lessundesired electromagnetic radiation as compared with the addition of theextra equivalent lengths. The easiest preferable way to implement thevaried void opening patterns 354, 356, 358 is to design them withcompatible percentage of void open areas for varied length oftransmitted lines 290, 292, 294, 296, 298, 300. As shown in FIGS. 14 and15, the longer the length of the transmission lines 290-292 is, the lessthe percentage of void opening square pattern 354 is. That is, the lessthe percentage of void area of square pattern 354, the less transmittedtime of the signal conductors 290-292 is. If the accuracy of impedanceis also required in the above varied void opening area, then differentand compatible widths of the signal lines for varied length of thetransmission lines are needed to have the same impedance. Therefore, thelonger the length of the transmission lines 290, 292 with smallerpercentage of void area square pattern 354 is, the less the width of thesignal lines 290, 292 is needed to have the same impedance. It impliesthe width of signal conductors 290, 292 is less than that of signalconductors 294, 296 as shown in FIGS. 15, 16.

FIG. 17 depicts a plan view of a shielded flexible circuit board 372 asthe fifth embodiment in accordance with the present invention, whereinthe circuit board 372 has varied square void openings with locationconfiguration which is related to the curve of the circuit board 372.FIG. 18 depicts an enlarged view of a section denoted by 18 in FIG. 17.FIG. 19 depicts a cross-sectional elevational view of the presentinvention taken along the line 19—19 in FIG. 18. It should be noted thatvoid opening square patterns 460, 462, 464 have varied opening squares.Varied void opening square patterns 460, 462, 464 induce slow waveeffects to compensate for timing effect with less undesiredelectromagnetic radiation as compared with the addition of the extraequivalent lengths. These lines 390,392, as shown in FIG. 18, are neededto connect different sizes of void opening square patterns 460, 462, 464for better shielding effect. The easiest preferable way to implement thevaried void opening patterns 460, 462, 464 is to design them withcompatible percentage of void open areas for varied length oftransmitted lines 360, 362, 364, 366, 368, 370. As shown in FIGS. 17 and18, the longer the length of the transmission lines 360, 362 is, theless the percentage of void opening square pattern 460 is. That is, theless the percentage of void area of square pattern 460 and the less thetransmitted time of the signal conductors 360, 362 is. If the accuracyof impedance is also required in the above varied void opening area,then different and compatible widths of the signal lines for variedlength of the transmission lines are needed to have the same impedance.Therefore, the longer the length of the transmission lines 360, 362 withsmaller percentage of void area square pattern 460 is, the less thewidth of the signal lines 360, 362 is needed to have the same impedance.It implies the width of signal conductors 360, 362 is less than that ofsignal conductors 364, 366 as shown in FIGS. 18, 19.

Although only the preferred embodiments of this invention were shown anddescribed in the above description, it is requested that anymodification or combination that comes within the spirit of thisinvention be protected.

What is claimed is:
 1. A circuit board comprising: a first predeterminedshielding configuration, defined in a first plane, comprising a firstset of predetermined varied void opening patterns with a firstpredetermined location configuration which comprises locations of saidfirst set of predetermined varied void opening patterns; a secondpredetermined shielding configuration, defined in a second plane,comprising a second set of predetermined varied void opening patternswith a second predetermined location configuration which compriseslocations of said second set of predetermined varied void openingpatterns; and signal conducting elements being disposed between saidfirst and second planes and having a virtual ground of differential modepair of said signal conducting elements defined as the middle of saiddifferential mode pair of said signal conducting elements; wherein saidsignal conducting elements or said virtual ground is positioned at apredetermined location with respect to said first and secondpredetermined shielding configurations.
 2. The circuit board of claim 1,wherein said first and second sets of predetermined varied void openingpatterns comprise varied void openings and a largest dimension of saidvaried void openings is less than one-twentieth of a wavelength of ahighest frequency of a signal conducted by said signal conductingelements.
 3. The circuit board of claim 1, wherein said first and secondpredetermined shielding configurations comprise conductive elements withsaid predetermined varied void opening patterns.
 4. The circuit board ofclaim 3, wherein a combination of said first and second sets ofpredetermined varied void opening patterns, and a spacing of saidconductive elements with respect to said signal conducting elements isdesigned to match requirements of impedance, transmitted timing andelectromagnetic radiation control for said signal conducting elements.5. The circuit board of claim 3, wherein said conductive elements ofsaid first and second predetermined shielding configurations are thinfilm type conductive elements.
 6. The circuit board of claim 1, whereinsaid first and second predetermined location configurations comprisegrids.
 7. The circuit board of claim 1, wherein said first and secondpredetermined location configurations are aligned with respect toorientation curves of said signal conducting elements or said virtualground of differential mode pair of said signal conducting elements. 8.The circuit board of claim 1, wherein each void opening pattern of saidfirst and second sets of predetermined varied void opening patternscomprises a void opening pattern selected from the group of circle,ellipse, square, and rhombus patterns.
 9. The circuit board of claim 1,wherein said first and second sets of predetermined varied void openingpatterns comprise varied symmetric void opening patterns.
 10. Thecircuit board of claim 9, wherein said predetermined location of saidsignal conducting elements or said virtual ground with respect to saidfirst and second predetermined shielding configurations is determined bycenters of said symmetric void opening patterns.
 11. The circuit boardof claim 1, wherein said first and second predetermined shieldingconfigurations and said signal conducting elements comprise a flexiblecable.
 12. The circuit board of claim 1, wherein said first and secondplanes are connected and designated as only ground for said differentialmode of pair of said signal conducting elements, and a power, a groundfor power, or common mode signals are conducted by said signalconducting elements.
 13. The circuit board of claim 1, wherein saidsecond predetermined location configuration is identical to said firstpredetermined location configuration.
 14. The circuit board of claim 13,wherein said first and second predetermined location configurationscomprise grids.
 15. The circuit board of claim 1, wherein said firstpredetermined shielding configuration comprises varied circle voidopening patterns with a predetermined grid type location configuration,and said second predetermined shielding configuration is identical tosaid first predetermined shielding configuration.
 16. The circuit boardof claim 15, wherein said varied circle void opening patterns comprisevaried circle void openings and a largest dimension of said variedcircle void openings is less than one-twentieth of a wavelength of ahighest frequency of a signal conducted by said signal conductingelements.
 17. The circuit board of claim 15, wherein said first andsecond predetermined shielding configurations and said signal conductingelements comprise a flexible cable.
 18. The circuit board of claim 15,wherein said first and second planes are connected and designated asonly ground for said differential mode of pair of said signal conductingelements, and a power, a ground for power, or common mode signals areconducted by said signal conducting elements.
 19. The circuit board ofclaim 15, wherein said first and second predetermined shieldingconfigurations comprise thin film type conductive elements with saidvaried circle void opening patterns.
 20. The circuit board of claim 1,wherein said first predetermined shielding configuration comprisesvaried ellipse void opening patterns with a predetermined grid typelocation configuration, and said second predetermined shieldingconfiguration is identical to said first predetermined shieldingconfiguration.
 21. The circuit board of claim 20, wherein said variedellipse void opening patterns comprise varied ellipse void openings anda largest dimension of said varied ellipse void openings is less thanone-twentieth of a wavelength of a highest frequency of a signalconducted by said signal conducting elements.
 22. The circuit board ofclaim 20, wherein said first and second predetermined shieldingconfigurations and said signal conducting elements comprise a flexiblecable.
 23. The circuit board of claim 20, wherein said first and secondplanes are connected and designated as only ground for said differentialmode of pair of said signal conducting elements, and a power, a groundfor power, or common mode signals are conducted by said signalconducting elements.
 24. The circuit board of claim 20, wherein saidfirst and second predetermined shielding configurations comprise thinfilm type conductive elements with said varied ellipse void openingpatterns.
 25. The circuit board of claim 1, wherein said firstpredetermined shielding configuration comprises varied rhombus voidopening patterns with a predetermined grid type location configuration,and said second predetermined shielding configuration is identical tosaid first predetermined shielding configuration.
 26. The circuit boardof claim 25, wherein said varied rhombus void opening patterns comprisevaried rhombus void openings and a largest dimension of said variedrhombus void openings is less than one-twentieth of a wavelength of ahighest frequency of a signal conducted by said signal conductingelements.
 27. The circuit board of claim 25, wherein said first andsecond predetermined shielding configurations and said signal conductingelements comprise a flexible cable.
 28. The circuit board of claim 25,wherein said first and second planes are connected and designated asonly ground for said differential mode of pair of said signal conductingelements, and a power, a ground for power, or common mode signals areconducted by said signal conducting elements.
 29. The circuit board ofclaim 25, wherein said first and second predetermined shieldingconfigurations comprise thin film type conductive elements with saidvaried rhombus void opening patterns.
 30. The circuit board of claim 1,wherein said first predetermined shielding configuration comprisesvaried square void opening patterns with a predetermined grid typelocation configuration, and said second predetermined shieldingconfiguration is identical to said first predetermined shieldingconfiguration.
 31. The circuit board of claim 30, wherein said variedsquare void opening patterns comprise varied square void openings and alargest dimension of said varied square void openings is less thanone-twentieth of a wavelength of a highest frequency of a signalconducted by said signal conducting elements.
 32. The circuit board ofclaim 30, wherein said first and second predetermined shieldingconfigurations and said signal conducting elements comprise a flexiblecable.
 33. The circuit board of claim 30, wherein said first and secondplanes are connected and designated as only ground for said differentialmode of pair of said signal conducting elements, and a power, a groundfor power, or common mode signals are conducted by said signalconducting elements.
 34. The circuit board of claim 30, wherein saidfirst and second predetermined shielding configurations comprise thinfilm type conductive elements with said varied square void openingpatterns.
 35. A circuit board comprising: a first predeterminedshielding configuration, defined in a first plane, comprising a firstset of predetermined varied void opening patterns with a firstpredetermined location configuration which comprises locations of saidfirst set of predetermined varied void opening patterns; and signalconducting elements being disposed in a plane parallel to said firstplane and having a virtual ground of differential mode pair of saidsignal conducting elements defined as the middle of said differentialmode pair of said signal conducting elements; wherein said signalconducting elements or said virtual ground is positioned at apredetermined location with respect to said first predeterminedshielding configuration.
 36. The circuit board of claim 35, wherein saidfirst set of predetermined varied void opening patterns comprises variedvoid openings and a largest dimension of said varied void openings isless than one-twentieth of a wavelength of a highest frequency of asignal conducted by said signal conducting elements.
 37. The circuitboard of claim 35, wherein said first predetermined shieldingconfiguration comprises a conductive element with said first set ofpredetermined varied void opening patterns.
 38. The circuit board ofclaim 37, wherein a combination of said first set of predeterminedvaried void opening patterns, and a spacing of said conductive elementwith respect to said signal conducting elements is designed to matchrequirements of impedance, transmitted timing and electromagneticradiation control for said signal conducting elements.
 39. The circuitboard of claim 37, wherein said conductive element of said firstpredetermined shielding configuration is a thin film type conductiveelement.
 40. The circuit board of claim 35, wherein said firstpredetermined location configuration comprises grids.
 41. The circuitboard of claim 35, wherein said first predetermined locationconfiguration is aligned with respect to orientation curves of saidsignal conducting elements or said virtual ground of differential modepair of said signal conducting elements.
 42. The circuit board of claim35, wherein each void opening pattern of said first set of predeterminedvaried void opening patterns comprises a void opening pattern selectedfrom the group of circle, ellipse, square, and rhombus patterns.
 43. Thecircuit board of claim 35, wherein said first set of predeterminedvaried void opening patterns comprises varied symmetric void openingpatterns.
 44. The circuit board of claim 43, wherein said predeterminedlocation of said signal conducting elements or said virtual ground withrespect to said first predetermined shielding configuration isdetermined by centers of said symmetric void opening patterns.
 45. Thecircuit board of claim 35, further comprising a second predeterminedshielding configuration, defined in a second plane, comprising acombination of a second set of predetermined void opening patterns witha second predetermined location configuration which comprises locationsof said second set of predetermined void opening patterns, and a secondset of predetermined solid patterns positioned at predeterminedlocations.
 46. The circuit board of claim 35, further comprising asecond predetermined shielding configuration, defined in a second plane,comprising a second set of predetermined varied void opening patternswith a second predetermined location configuration which compriseslocations of said second set of predetermined varied void openingpatterns, said second set of predetermined varied void opening patternsbeing divided into a plurality of groups connected by conductive lines,each group comprising a plurality of identical void openings.